1. Field of the Invention
The present invention pertains to an improved dynamic range dimmer for a gas discharge lamp. In particular, the present invention relates to a dimmer, for a gas discharge tube, which provides sufficient voltage to sustain gas discharge tube arc during manual dimming and accommodates aging of the gas discharge tube.
2. Description of the Prior Art
The impedance characteristics of gas discharge lamps are both non-linear and a negative resistance, such that current flowing through the lamp is not directly proportional to the voltage supplied to the lamp. Until a minimum voltage is reached, the current through the gas discharge lamp is zero. The lamp conducts only after the minimum voltage is achieved. Ballasts are connected in series with the gas discharge lamps to limit the current, which would otherwise increase rapidly and unchecked.
If a load in an alternating current (AC) circuit is equivalent to a pure resistance then the circuit would have a power factor of one. Such a power factor of one is preferred and means that the voltage and current are sinusoidal and in phase with one another.
Dimming of gas discharge lamps is difficult due to the non-linear characteristics of gas discharge lamps. It is highly desirable to vary the light intensity of gas discharge lamps over a large range. A range of least 10:1 is required in most situations, with a range of 20:1 being preferred. One reason for having a large range is that the eye is a non-linear device, especially at high light levels, and a change of 30% is barely perceptible.
Another reason a large range is desired is to save energy. It is preferable to run the gas discharge lamps at very low levels when a room is unoccupied. Most existing designs barely reach the 10:1 ratio, and very few of those exceed it. Yet another problem with existing gas discharge lamp configurations is if they have a dimming capability, when they are in a dimmed condition and the power is interrupted, the gas discharge lamp cannot be restarted, or is very difficult to restart until it is restored to the undimmed state.
Various methods of dimming have been developed, all of which limit power to the gas discharge lamp. These approaches can be divided into three types. First, one can control the amplitude of the energy fed into the lamp via its external circuitry. This can be achieved by voltage or current limiting at either AC or DC inputs. If the ballast is electronic and has an inverter, the AC or DC levels can be limited. Second, one can control the duty cycle or "on time" of the AC; again either internally or at the input. There are various methods to control the duty cycle, for example, pulse width modulation phase control, etc. Third, in electronic ballasts the frequency can be varied.
To ensure proper gas discharge tube startup, the frequency of the driving oscillator must match the resonant frequency of the series resonant output circuit. At resonance the voltage developed across the output circuit reaches peak value. The amount the peak value is above the driver input voltage is dependent upon the quality factor or Q, of the output circuit. The peak voltage must be sufficient to initiate the arc in the gas discharge tube. Once the tube arc has started, it is no longer necessary to develop the peak voltage. Furthermore, maintaining operation of the resonant frequency of the output circuit will cause a loss of power in the series resonant output circuit.
In view of the foregoing, it is an object of the present invention to provide a dimmer for gas discharge lamps which has a large range of dimming.
Another object of the present invention is to provide a dimmer for gas discharge lamps in which the gas discharge lamp will resume emitting light at a dimmed level, after the light is dimmed and the power is subsequently interrupted.
A further object of the present invention is to provide a dimmer for gas discharge lamps which frequency shifts in the start mode and employs frequency shifting and variation of the duty cycle to accomplish dimming.